1. Field of the Invention
The invention relates to an apparatus for connection of workpieces using the friction stir welding method with a shaft which can be driven such that it rotates and at whose end remote from the drive end of the shaft a pin-like projection is arranged, at whose end a first stop, which is formed by a first shoulder, is arranged, with the first shoulder having a diameter which is larger than the diameter of the pin-like projection, and with a second stop, which is formed from a second shoulder and is arranged such that the workpieces to be connected can be enclosed between the stops, in that at least one of the stops can be moved translationally in order to enclose the workpieces with a predetermined force in the direction of the other stop.
2. Discussion of Prior Art
An apparatus of this type is known (EP-B-0 615 480 and DE-C-199 57 136). Friction stir welding, a further development of friction welding and also widely known as FSW, has fundamentally been known for several years and has repeatedly been developed further.
Originally, friction welding was carried out by moving two workpieces which are intended to be connected to one another by friction welding against one another in the desired connecting area, pressing them against one another with a force which can be preset in the process. The heat created by the friction in the end results in the material of the workpieces being plasticized in the connecting area. Once the material has been sufficiently plasticized, adequate thorough mixing of the materials of the two workpieces can take place at least in the area of the connection close to the surface, so that the desired welded joint is formed between the two workpieces as they cool down.
In the case of friction stir welding, there is no need for any relative movement between the workpieces in order to produce the friction and the thorough mixing of the materials. Instead of this, a pin-like projection or a cylindrical projection, which is caused to carry out a sufficiently large rotation by a drive or a motor, is placed against the end area of two workpieces which are to be connected and are located such that they abut against one another or overlap one another. With suitable guidance, as can be provided, for example, by means of a specific guide apparatus or else by a robot, the pin-like projection is additionally caused, for example, to carry out a translational movement along the abutting edges of the two workpieces to be connected. The workpieces are prevented from escaping from one another by means of a robust, static opposing bearing.
Once the material of the workpieces has been sufficiently plasticized after the start of the welding process by the friction heat that is produced in the adjacent material area as a consequence of the rotation of the pin-like projection with the material of the workpieces, the translational movement is carried out along the bead profile between the two workpieces while maintaining the rotational movement of the pin-like projection, thus forming, for example, a longitudinal bead.
With regard to the apparatus of this generic type according to EP-B-0 615 480, the workpieces are held together by means of the known apparatus in the area around the abutting edge and the weld bead that is formed by means of two stops with a larger diameter than the pin-like projection, by the pin-like projection being enclosed between the two stops. Those faces of the two stops which face one another effectively form shoulders which each cover the surfaces of both workpieces to be connected in a rotating form on one face of the workpieces around the area of the weld bead that is to be formed. If, by way of example, pressure is exerted by means of the apparatus orthogonally with respect to the surface of the two workpieces to be connected, the contact pressure on the side of the workpieces to be connected which faces away from the pressure is reduced, corresponding to the shoulder there on the basis of the rigid separation between the two shoulders of the two stops. For this reason, special pressure means must be used with this apparatus in order to provide a suitable opposing bearing, as still possible with acceptable complexity for workpieces such as metal sheets and the like which are flat or in the form of panels, but is normally impossible for complicated welded joints produced by means of the friction stirring method owing to the complicated shapes of the workpieces.
Furthermore, industrial robots are used for a wide range of functions in many manufacturing areas, for example for motor vehicle construction or aircraft construction, in which it is not only difficult but even often completely impossible to provide flat or other opposing bearings for producing the welded joint and, furthermore, the robots themselves cannot also produce the required pressure forces, or can do so only in a very highly complex manner.
DE-C-199 57 136 discloses an apparatus in which at least one of the stops for carrying out the welding process can be moved under the influence of the workpieces and can be enclosed with a force that can be predetermined. This makes it possible for the apparatus itself to apply the necessary pressure to both faces of the workpieces to be connected without any opposing bearing being required. Thus, even in the case of complicated workpieces, it is possible to produce weld beads, for example weld beads which run in three dimensions in space, without any substrate being required, which in the past would have had to secure the root of the weld bead, and would at the same time have had to support the workpieces. This makes it possible to avoid the handling system having to apply the force to the workpieces to be connected.
In order to achieve this, a linear-movement cylinder is provided on the apparatus, connected to the rotation shaft, which linear-movement cylinder produces a translational movement when a hydraulic medium is applied appropriately, thus applying the force to the workpieces to be connected.
Furthermore, a first stop is provided, which can rotate, is connected to the rotation shaft, presses against the workpieces to be connected from underneath as a result of the translational movement, and clamps the workpieces against a second, fixed stop.
The fixed upper second stop and the rotating lower first stop result in different temperatures on the left-hand side and right-hand side of the workpieces to be connected during the friction stir welding process, owing to the flowing behavior of the material. This results in different heat distributions within the workpieces to be connected, and this can influence the quality of the weld bead. This is particularly problematic in the case of thin metal sheets, because the resultant temperature difference is very great, because it is impossible to produce a sufficiently great thermal flow to the boundary surfaces by convection. In consequence, the characteristics of the metal sheets may be adversely affected or even destroyed in the weld area, so that it is no longer possible to produce a joint. Furthermore, the temperature difference that exists can influence the quality of the bead.